1. Field of the Invention
The present invention relates to a rubber composition for a tire tread and a pneumatic tire using the rubber composition.
2. Description of the Related Art
In general, a tread rubber composition for a studless tire is required first to improve in a braking performance on ice. However, it is required to improve concurrently in abrasion resistance, tear resistance and crack resistance.
A tire performance on ice can be improved by application of foamed rubber and a tread pattern technique, but in these methods the cell fraction and the edge parts have to be increased, respectively, and these methods lead to a reduction in the abrasion resistance, the tear resistance and the crack resistance.
Further, a method in which glass beads, rice hulls and walnut husk pieces are mixed to raise the performance on ice is known as well, but these walnut husk pieces and the like act as breaking nuclei in a rubber composition to result in reducing the abrasion resistance, the tear resistance and the crack resistance.
A method in which a large amount of high grade carbon black is blended in rubber is known as a method for improving abrasion resistance of a tire, but this method brings about an increase in the hardness of the rubber composition and a reduction in the tensile elongation at break and leads to a reduction in the tear resistance and the crack resistance.
Further, if the tear resistance and the crack resistance are intended to be improved by using rubber composition having specific physical properties, application of any of foaming, foreign materials and high grade carbon black brings about an adverse effect, and therefore the existing situation is that only a device on a tread pattern design is adopted.
In addition thereto, a method in which polyethylene is blended is known as a method for improving the abrasion resistance, the tear resistance and the crack resistance.
For example, a rubber composition which comprises 100 parts by weight of a rubber component blended with 2 to 75 parts by weight of high density polyethylene (20% by weight of high density polyethylene having cross-linkable parts is included therein) and which is kneaded at a temperature higher than the melting point of the blended resin is proposed in Japanese Patent Application Laid-Open No. Hei 10-67886 which is a prior application filed by the present asignee. Polyethylene C (Comparative Examples 9 and 11) shown in Table 1 has a melting point of 124.degree. C. and a density of 0.923, and it is disclosed that the rubber compositions (Comparative Examples 9 and 11) blended with this polyethylene C fails in targeted hardness and has a large permanent set.
Further, proposed in Japanese Patent Application Laid-Open No. Hei 9-254606 is a pneumatic tire for a heavy load in which a rubber composition comprising a rubber component having a specific composition blended with a polyethylene having a melting point of 120.degree. C. or higher (135.degree. C. in the examples) and carbon black having specific physical properties in specific amounts is used for a tread rubber in which the abrasion resistance is allowed to be compatible with the rupture resistance.
Further, known as pneumatic tires blended with polyethylene are, for example, a pneumatic tire disclosed in U.S. Pat. No. 4,675,349, wherein a polyethylene having a softening point of 135.degree. C. or higher is blended at a temperature lower than the softening point and a pneumatic tire disclosed in U.S. Pat. No. 5,341,869, which uses low density polyethylene (LDPE) whose crystal has a melting point falling in a range of 104 to 115.degree. C.
However, it is neither described nor suggested in these official gazettes to apply polyethylene (PE) to a foamed rubber. In addition, investigations made by the present inventor have revealed the problem that when applying PE to a foamed rubber, a cell fraction of the vulcanized rubber composition becomes unstable.
Further, there are the problems that the viscosity of the rubber composition increases at an extruding step, where PE and a foaming agent are blended, to reduce the workability, and that because the temperature is elevated, foaming starts locally to consume a part of the foaming agent, so that the cell fraction of the finished tire becomes unstable and the performance on ice, which is the original object, is reduced. The higher the softening point (120 to 130.degree. C.) of the resin compared with the extrusion temperature (100 to 110.degree. C.), the more this fact is notable. In general, high density PE has the drawback that the blending thereof brings about an increase in the viscosity of the rubber composition at a working temperature because of the high melting point thereof to deteriorate the extrusion workability, and low density PE causes the problem that the tire is inferior in durability.